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Home > Family > Dad's Parenting Style May be Predicted By Size of His Testicles (STUDY)

Summary: Size does matter — men with smaller testicles are more involved in bringing up baby.

By Joanne Van Zuidam | Posted: September 11, 2013

Who's got big balls? According to a new study, fathers that are less involved. That's who.

Researchers at Emory University found guys with large testicles have a more hands-off parenting style while guys with small testicles are more nurturing and involved.

The study was published in the Proceedings of the National Academy of Sciences.

In looking at why some dads are more involved than others, the researchers tested the theory that people and animals are either built to breed or to nurture.

To test that theory, scientists had study participants and their female partners fill out separate questionnaires about daily parenting involvement, such as changing diapers, feeding, and caring for a sick child. MRI scans analyzed brain activity in men while being shown picture of their toddler and also strangers' children. The volume of the men's testes was measured in a voluntary MRI scan (though only 55 of the 70 men involved in the study agreed to that test.)

The small study involved 70 men ages 21-55. The men, of varying ethnicities, all had children aged one to two, and lived with the biological mothers of their children. Most were married.

The results showed men with bigger testes had a less-involved parenting style, and the reward centers of their brains activated less when viewing their children's pictures. In comparison, men with smaller testes reported more daily involvement in raising their children and showed more nurturing activity in the brain when shown their children's pictures.

"I wouldn't want to say that men with large testes are always bad fathers but our data show a tendency for them to be less involved in things like changing diapers, bathing children, preparing meals, taking them to the doctor and things like that," said lead author James Rilling, an associate professor of anthropology.

As reported in the NY Daily News, previous studies have shown a link between high testosterone levels and lower parental involvement as well as divorce and infidelity. The research team also analyzed testosterone levels and found the same inverse relationship to parental involvement in their study.

"Other people have looked at testosterone and parental behavior but as far as we know we are the first to look at testes size and parental behavior and we think we are getting at something different," said Rilling.

"We are suggesting that men with larger testes are more built for a mating effort strategy and as a consequence are less built for investing in children."

The research didn't determine whether testes size is to blame for the difference in parenting style, or if the act of becoming a more-involved caregiver may have caused the testes to shrink in some men, which happens with age and with environmental influences (like becoming a dad).